The invention relates to a process for producing lightweight, panel- or block-shaped gypsum building materials, in particular wall panels provided with a porous structure, wherein calcium sulphate alpha-hemihydrate, water in a slightly more than stoichiometric quantity and, if appropriate, setting retarders and/or accelerators for gypsum and additives are mixed to form a pourable suspension and formed into a desired shape.
German Offenlegungsschrift 1,571,575, describes a process for producing gypsum building materials in which unground calcium sulphate alpha-hemihydrate is used as gypsum, with which a pourable suspension is formed, to which calcium carbonate is added and reacted with sulfuric acid to produce carbon dioxide. The gas bubbles thus generated in the suspension lead to a pore structure in the finished product. Such a generation of gas bubbles in the suspension, however, leads to problems with respect to a uniform distribution thereof over the cross-section, in particular since the gas bubbles tend to rise and the gypsum particles tend to sediment, so that the quality of the porous gypsum products in impaired. To reduce this problem, the sulfuric acid is added immediately before pouring, so that the gas bubbles form substantially in the poured suspension, which thus expands in the mould. In addition, it is pointed out that preformed foams do not give good results in this connection, since they retard setting and thus have time to collapse, and the viscosity is impaired.
According to the German Offenlegungsschrift 2,442,021, anhydride is used as gypsum, while gas is generated in the suspension produced by catalytic decomposition of hydrogen peroxide. The porosified suspension is poured into moulds before the maximum expansion has been reached. Apart from the fact that anhydride does not lead to strengths as high as those obtained with calcium-sulphate alpha-hemihydrate, problems here again result from the rising of gas bubbles and sedimenting of gypsum particles.
In addition, it is known from German Offenlegugngsschrift 2,546,181 to add a foaming agent to a suspension of gypsum, water and additives in the presence of a foam-stabilizing additive and to foam up the mixture. Such foaming-up, however, does not lead to a substantially uniform and maintainable mean foam pore size but, instead, these sizes fluctuate within a wide range, so that there are pores from sink hole size down to fine pores, whereby the density and quality of the foamed gypsum products produced are impaired.
It is known from German Offenlegungsschrift 2,740,018 to use calcium sulphate alpha-hemihydrate together with a proportion of dihydrate and to foam up the suspension using an added foam former, the dihydrate being intended to prevent a coalescence of foam bubbles. Since, however, the foam is generated in the suspension, a well defined pore size and number of pores cannot be set, with the result that the end products show corresponding fluctuations in density and quality.
Finally, it is known from German Offenlegungsschrift 2,548,912 to prepare, in a mixer, an aqueous surfactant foam of a structure which is complicated due to the use of chemicals used in addition to the surfactant, gypsum in the form of, for instance, hemihydrates then being added to the foam. The additional chemicals are intended to serve for stabilization of the foam.